This patent discloses a method (Contact Ninhydrin) of using a suitable absorbent substrate, super-absorbent polymer or other moisture source, and a chemical combination on card stock, containing Ninhydrin and enhancers, to visualize latent fingerprints on the absorbent surfaces (paper). This method utilizes microwave energy in combination with the enumerated components to provide a faster, easier and more reliable method than prior wet chemical methods. Further, minimally trained or lay persons can use the proposed method to obtain high quality and easily observed fingerprints on a variety of absorbent surfaces using no organic solvents.
Visualizing latent images left on absorbent surfaces, most commonly paper, is an important investigative technique used by investigators and fingerprint examiners. For many years a solution of Ninhydrin in acetone was the reagent of choice for visualizing latent prints on absorbent surfaces, since Ninhydrin is quite soluble in acetone and one can easily dissolve more than enough Ninhydrin to make a useful working solution. It is currently more common to dissolve the Ninhydrin in a nonflammable solvent or a hydrocarbon solvent with other additives to help the Ninhydrin dissolve since Ninhydrin is most soluble in polar solvents. The chemical reaction between Ninhydrin and amino acids contained in the fingerprint residue is relatively slow at room temperature. People have tried to accelerate the process by steaming or ironing the latent fingerprint substrate after it was dipped in a Ninhydrin solution to make the reaction go more quickly.
A disadvantage to this as it frequently causes purple color development in the background of the substrate and thereby reduces contrast with the purple color that is formed when amino acids react with the Ninhydrin solution. Therefore, although the reaction proceeds more quickly the fingerprints visualized may not be as highly visible. Polar solvents and also sometimes even many non-polar solvents, will cause problems for Questioned Document examiners because the solvent that is used for the Ninhydrin solution will often cause ink, particularly ballpoint pen ink, to run and one loses clarity in the document. Although commercial or lab prepared Ninhydrin solutions are available, many investigators and fingerprint examiners are not chemists and not comfortable with proper precautions and safety measures when using organic solvents both polar and non-polar.
A little used method that was called “Dry Ninhydrin” has been known for a number of years to avoid the ink running problem. This was done by dipping pieces of paper in a Ninhydrin solution and then, after drying the paper, applying them, usually to both sides of an evidence document, since it is not known which side of the substrate fingerprints are on, and then placing the combination under some weight to improve contact. This combination is then allowed to react in the dark, usually for an extended period of time. One must make these dry Ninhydrin materials to use with the “dry Ninhydrin” method since, as far as we know, they are not available commercially. This method has never been widely used since it is very slow, a minimum of 48 hours is recommended and results are not as reliable as the solution method. We thought we could improve this method by warming or finding a catalyst. Surprisingly, we discovered that if in addition to the Ninhydrin sheets we added a diaper pad that had been dampened to wet the super-absorbent polymer beads in the diaper pad and warm the package, under tension, in a microwave oven one obtained good color development on the substrate with gently heating in about five minutes. We have greatly optimized this novel process as described in the
Although commercial or lab prepared Ninhydrin solutions are available, many fingerprint examiners are not chemists and not comfortable with proper precautions and safety measures in using organic solvents both polar and non-polar. A method called “Dry” or as we now prefer to refer to it as “Contact” Ninhydrin has been used for a number of years to avoid the ink running problem. This was done by dipping absorbent pieces of paper in a Ninhydrin solution and then, after the solvent had evaporated, placing the Ninhydrin paper usually on both sides of a evidence document, since it is not known which side of the substrate fingerprints are on, and then placing the combination between two objects and placing them under something heavy to promote contact. The package is then allowed to react in the dark, usually for an extended period of time (forty eight hours or more). One must make these dry Ninhydrin materials to use with the “dry” Ninhydrin method since, as far as we know, they are not available commercially. There are several disadvantages to the “dry” Ninhydrin method as it has been used in the past.